The present invention generally relates to surface mount electronic devices. More particularly, this invention relates to a semiconductor device having a micromachine and capable of being surface mounted as a package to a circuit board.
A variety of semiconductor micromachines are known, including yaw (angular rate) sensors, angular and linear accelerometers, pressure sensors, thermal sensors, and actuators such as nozzles and valves. Each of these devices typically involves one or more micromachined structures (micromachines) formed in or on a silicon chip (referred to herein as a device chip). The device chip is often placed within a protective subpackage and then wire bonded to electrically connect the device to bond pads on the subpackage. The bond pads of the subpackage can then be reflow soldered to conductors on a circuit board, electrically and physically interconnecting the device to the board circuitry. Alternatively, device chips can be glued to a ceramic substrate, and then wire bonded to a circuit board after other surface mount components have been reflow soldered to the board.
Another packaging alternative involves wafer bonding methods, in which the micromachine of a device chip is enclosed by a second chip (referred to herein as a capping chip), which is bonded to the device chip. A cavity is often formed in the capping chip to receive and/or provide clearance for the micromachine of the device chip. Absolute pressure sensors require that the cavity be evacuated and hermetically sealed, while the performance of yaw sensors and accelerometers with resonating and tunneling micromachines generally benefit if the cavity is evacuated so that the micromachine operates in a vacuum. Bonding is typically achieved by forming the capping chip of silicon or glass (e.g., Pyrex), which can be bonded to the silicon device chip by such known techniques as anodic bonding and silicon fusion bonding, or with the use of glass frit, adhesives and solder. An example of this method is represented in FIG. 1, in which a micromachine sensor 110 is shown to include a device chip 112 with a surface micromachine 114, and a capping chip 116 with a cavity 118 in which the micromachine 114 is received. A portion of the capping chip 116 is removed by cutting or etching to allow for wire bonding of bond pads 120 on the device chip 112 to a ceramic substrate (not shown) to which the sensor 110 is attached by glueing or another suitable method. The substrate is then placed in a cavity package and mounted to a circuit board.
From the above, it can be appreciated that semiconductor micromachines have required various packaging and bonding steps that add significant cost. Accordingly, it would be desirable if semiconductor micromachines could be produced and packaged with reduced material and processing requirements, yet were produced in a form that protects the delicate micromachine from potential hazards within its operating environment.
The present invention is directed to a semiconductor device and method by which a device chip with a micromachine is directly surface mounted to a circuit board. Semiconductor devices in accordance with this invention generally entail a device chip with a micromachine and electrically-conductive runners that electrically connect the micromachine to appropriate signal conditioning circuitry. A capping chip is bonded to the device chip and encloses the micromachine. The capping chip has a first surface facing the device chip, an oppositely-disposed second surface, and electrical interconnects through the capping chip between the first and second surfaces. The electrical interconnects electrically communicate with the runners on the device chip, thereby providing a signal path from the micromachine to the exterior of the device. The capping chip further includes bond pads in electrical communication with the electrical interconnects. With the bond pads, the capping chip can be surface mounted to a circuit board by reflowing solder bumps formed on the bond pads. Depending on the placement of the bond pads on the capping chip, the semiconductor device can be mounted to the circuit board with the capping chip between the device chip and circuit board, or the semiconductor device can be mounted with one side of the device attached to the circuit board.
The method of this invention generally entails forming the device and capping chips in accordance with the above, and then bonding the capping chip to the device chip so as to enclose the micromachine within the semiconductor device and electrically connect the micromachine to the bond pads on the exterior of the capping chip. Bonding is preferably performed with solder bumps formed on the capping chip. The solder bumps are located on the capping chip so as to register with the runners on the device chip when the capping and device chips are mated. Reflowing causes the solder bumps to form solder connections that physically interconnect the runners to the electrical interconnects, and thereby electrically interconnect the micromachine to the bond pads of the semiconductor device.
In view of the above, a semiconductor device with a micromachine element can be manufactured and surface mounted to a circuit board without the additional steps of wire and adhesive bonding, without a chip for the sole purpose of enclosing the micromachine, and without a subpackage or cavity package to protect the micromachine.
Other objects and advantages of this invention will be better appreciated from the following detailed description.